Milk is rich in nutrients, and hence is prone to rapid spoilage, including microbial spoilage. Milk spoilage is associated with an increase in bacterial populations, microbial activity, or enzymatic activity. Spoilage can occur when enzymes produced by micro-organisms degrade nutrients in milk such as carbohydrates, proteins, and fats, producing undesirable end products. Further, because of the degradation of constituents, such milk becomes more susceptible to microbial contamination.
Microbial or enzymatic activity can lead to the production of lactic acid. The increase in acidity can cause precipitation and coagulation of proteins such as casein, causing curd formation. Once curd formation is initiated, the entire amount of milk can curdle within 24 hours. In a similar manner, spoilage in other liquids foods or substantially liquid food, for instance, fruit juices or soy milk, occurs due to microbial and enzymatic activity.
In developing countries, millions of small-scale farmers depend on only one or two dairy animals to supplement their livelihood. These farmers typically can collect only small amounts of milk from the animals they own. They rely on co-operative collection centers to purchase the milk on a daily basis. The farmers often travel significant distances to transport milk from their source to co-operative collection centers, where the milk is refrigerated. Multiple trips may be needed every day. Further, in many countries, the rate of spoilage of milk is higher due to the tropical climate, which tends to encourage microbial activity because of higher ambient temperatures. Most of the subsistence farmers in such countries cannot afford refrigeration. This results a significant decrease in milk production due to spoilage.
The typical amount of time required to transport the milk from the farmer to the collection center can range from 2 to 12 hours or even more. Milk that is not refrigerated can begin to spoil because of microbial or enzymatic activity, leading to undesirable changes such as curdling. Once curdling begins, a large volume of milk is spoiled and cannot be recovered.
Developing countries in particular produce more than 300 million tons of milk per year. However, due to the inefficient process, approximately 10% of the milk production is lost due to milk spoilage in countries that face electricity shortages in rural areas. This translates to several million dollars worth of milk that spoils due to less than optimal conditions.
Even when refrigeration is available as a method for preservation, milk often has a limited shelf life. Techniques like pasteurization and ultra-high-temperature (UHT) processing can extend the life of milk. However, pasteurized milk still needs to be refrigerated to prolong its keeping quality. UHT processed milk does not need refrigeration only till the integrity of the package is maintained. Once the package or container of UHT milk is opened, it needs to be refrigerated.
Thus, the shelf-life of processed milk often depends on refrigeration facilities available. In case of any disruption such as equipment failure or power failure that prevents refrigeration, milk spoilage is hastened.
Though chemical additives can be used to preserve beverages like fruit juices, no additives can be used to preserve milk because of government regulations and consumer preferences.
Calves are often fed milk that is kept at ambient temperatures in feeding pans. Due to risk of spoilage of milk, the feeding pans are required to be replenished 2-3 times in a day which becomes cost prohibitive. Even during this time there is rapid growth of pathogenic bacteria in stored milk which can be harmful to calves. If the shelf-life of milk/milk replacers in calf feeding pans can be increased and the growth of bacteria can be inhibited, it would improve calf management practices.
Hence a need exists for a low-cost technology for the preservation of milk that does not rely on refrigeration or additives. It is additionally desirable that alternative technology developed to preserve milk or extend its shelf life does not require alterations to current storage, transport, or processing equipment.